Method for evaluating and controlling a radar installation

ABSTRACT

A method is disclosed for evaluating the terrain surrounding a radar site. The method comprises to calculate the radar horizon around a radar site from stored terrain elevation information. The information obtained can be used for controlling the scanning profile of the radar, by letting the radar scan above the calculated horizon, and thus avoiding transmitting into the terrain.

FIELD OF THE INVENTION

The present invention relates to the field of radar technology, and inparticular a method for the calculation and presentation of a terrainprofile for the purpose of evaluating a geographical site, and controlthe scanning pattern of a radar from the terrain profile.

TECHNICAL BACKGROUND

When positioning a land-based radar which purpose is to scan the horizonaround a geographical site, it is often practical to know at whichelevation to start scanning. This way one can avoid spending timescanning directly into e.g. mountains.

One possible solution is for the radar to initiate a search in elevation(e.g. from the highest allowed elevation and down) for the horizon,typically on a sector-by-sector basis, and detect where the groundclutter level starts to be significant.

A radar can get a certain amount of false echo above the horizon (e.g.birds, second-time-around echo from distant planes, etc) which may setthe horizon higher than necessary. Sometimes radar echo are alsosufficiently absorbed by surrounding terrain so that the horizon may beset lower than required.

Rain, snow and fog can also affect such a measurement.

In addition a radar often has a minimum range of detection. This resultin objects which constitute part of the horizon and being “too close”blocks the view without the radar detecting this.

Another important issue in military applications is Electronic Warfarewhere radar silence is an important counter measure. This means thatsearch elevation have to be found without using the radar actively.

BRIEF SUMMARY OF THE INVENTION

Thus, it is an object of the present invention to provide a method forevaluating the terrain around a radar site that is quickly performedwithout relying on the radio properties of the radar installation.

Another object is to provide a method that can be performed withoutbetraying the presence of the radar.

These objects are met by a method as defined in the appended patentclaims. In essence, the method comprises to calculate the radar horizonaround a radar site from stored terrain elevation information.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail in reference to theappended drawings, in which

FIG. 1 is a diagram showing a sector of the terrain around a radar siteand the positions of the points in which the elevation of the terrain iscalculated.

FIG. 2 is a schematic view of a radar installation with the necessaryequipment for performing the inventive method.

FIG. 3 is a flow diagram showing the individual steps performed in theinventive method.

FIG. 4 is the resultant radar horizon in front views.

FIG. 5 is a section along one of the sectors in FIG. 4, illustrating howthe resultant profile is obtained.

DETAILED DESCRIPTION OF THE INVENTION

The invention consists of a method for evaluating a radar horizon arounda radar site. The radar can then import this information and use it forsetting up a scanning pattern that avoids the transmission of energyinto the surrounding terrain.

The method is performed on a computer that, from a given position,divides the surrounding terrain into sectors. In each sector a number ofcalculation points are chosen, based on simple geometricalconsiderations.

In each calculation point the radar search elevation angle is calculatedfrom terrain altitude information stored in a database. FIG. 2 shows theprocess. The computer consults the database for each calculation point.If the angle is greater than the previous angle in the current sector,then this angle is stored as the greatest angle. The reason for this isthat a nearby object, e.g. the small hill, might “shadow” a largerobject farther away, e.g. a mountain in the distance. The stored angleis lowest angle the sweeping ray may use in order to clear the terrainin this sector.

The suite of stored angles for all sectors considered constitutes a“radar horizon” for this particular site. The calculated horizon canthen be presented for the radar operator and the radar scanning patterncan be adjusted accordingly, either automatically (i.e. directly),manually (i.e. operator confirmed) or semi-automatically (i.e. directlybut where the operator can adjust manually at a later time).

The calculation is performed either as a separate program or as part ofan already existing program on the computer where access to a suitableset of terrain altitude data on a digital format is required. Thisterrain data, e.g. DTED, can be stored in a conventional or proprietarydatabase, preprosessed or otherwise, or in raw DTED format outside of,but connected to, the invention (DTED: Digital Terrain Elevation Data,METRIC MIL-PRF-89020A).

The latter is particularly ideal as it provides for the user of theinvention to add further terrain data with the detailing level for thearea of interest without requiring a separate tool.

FIG. 1 shows how the calculation points are chosen within a sector. Nearto the radar site, the altitude is fetched from the database in pointsalong a line going outward from the site. Whenever the distance betweenan edge of the sector and the nearest point grows too great the numberof calculation branches is increased by one. When branching occurs, thecalculated values in parallel points (i.e. in points at the same rangein parallel branches) are compared, and the highest value chosen as thevalue representing this particular range.

In particular the following parameters are considered:

-   Absolute geographical position of origo-   Sector width-   Number of sectors (ie. the total view)-   Azimuth offset-   Max sector range; the length of each sector from the observation    point-   Stepsize; the distance between each sampled point from the altitude    database (LSB in FIG. 1). The stepsize is chosen dependent on the    resolution in the altitude database and the general calculation    performance desired.

From these initial parameters the following is calculated:

-   Number of calculations (steps) in one path; this value is calculated    from the stepsize and sector width parameters in the input data.    Each look up in the altitude database is done for every step length    along a path (or a line) inside the sector until the stepsize limit    is reached. The stepsize limit is reached when the width of the    sector is greater than the horizontal cell size in the altitude    database.-   Number of branches; this value is calculated from the stepsize, max    sector range and stepsize parameters in the input data. When the    stepsize limit is reached for all existing parallel paths (if any)    then the algorithm performs a branch which means that the number of    parallels is increased by one.-   Number of parallel calculation paths; this value is calculated from    the stepsize parameter in the input data. When the look-ups in the    altitude database for the parallel paths do not cover the width of    the subsector, it is increased by one. The parallel paths have the    same azimuth and number of calculations on the path, but    displacement sideways is different.-   Displacement; a function of the previous parameter.

FIG. 3 shows visualizes the individual steps performed in the procedurefor evaluating a site.

A geographical point is calculated by looping through, in nested order,the number of branches, number of paralells and number of steps. Foreach point the following procedure is performed:

Based on the azimut, the range and the displacement a transformationfrom polar to Cartesian coordinates is performed. This gives ageographical coordinate relative to origo. For this point the terrainaltitude is retrieved from the height database (see FIG. 2). Thisaltitude is stored in the side-view terrain profile, if it is thehighest altitude for this range. The elevation is then calculated as:arctan (altitude/range)and is then compared to the previous highest elevation for the sector.If it is the highest it is stored in a front-view terrain profile, FIG.4.

This procedure is the repeated for all points and for all branches forthe specified sector. The end result can visually be presented like inFIG. 5.

The view in FIG. 5 is of the left-most sector in FIG. 4. The visibilityhere is limited by a nearby obstruction.

The invention can be used for evaluating possible radar sites, e.g. whenplacing a radar installation for an airport. It allows for off-siteevaluation of the terrain. Thereby multiple sites can be evaluated andthe best located.

The method can also be performed “on the fly” when relocating a mobileradar. In this application the vertical profile shown in FIG. 4 is ofparticular usefulness, as it can indicate for the operator that a betterplace can be found in the immediate neighborhood.

Another possible application is to use the method for automaticallyguiding a mobile unit, e.g. a vessel, car or airplane, to the “best”sites along its course.

However, the main application will be in establishing a scanning horizonfor a radar when it has been installed at its site. In this respect oneobtains the advantage of eliminating the problems of determining why anecho has not returned and whether it has returned for the “wrong”reasons.

1. A method for establishing a scanning horizon for a radar installationat a first position, comprising the steps of: a) dividing the terrainsurrounding said first position into a number of sectors, b) findingaltitudes of successive points along a sector by consulting terraininformation stored in a database, including near to the radar site,finding the altitudes in points along a line going outward from thesite, and whenever the distance between an edge of the sector and thenearest point grows above a predefined distance, increasing the numberof calculation branches by one, comparing the found values in parallelpoints, and choosing the highest value as the value representing thisparticular range, c) calulating an elevation value in each of thesuccessive points along said sector from the relationshipelevation=arctan (altitude/range), d) comparing the elevations of allthe successive points in the sector to find a point of highest elevatione) storing said point of highest elevation for said sector in a frontview terrain profile f) repeating steps b) to e) for another sector. 2.The method as claimed in claim 1, further comprising the step of storingthe altitudes found in step b) in a side view profile.
 3. An arrangementfor controlling a radar, comprising: a database containing terrainelevation information, means for fetching from the database thealtitudes of successive points along a sector of the terrain around theradar installation, means for calculating the elevations of thesuccessive points in the sector and comparing the elevations in order tofind the point of highest elevation, means for storing said point ofhighest elevation in a front view terrain profile, said means beingadapted to calculate corresponding information for all sectorssurrounding the radar installation, and a control unit adapted tocontrol said radar to scan the horizon defined by the elevations in saidfront view terrain table.
 4. The arrangement as claimed in claim 3,further comprising means for storing said altitudes in a side viewterrain profile.